1. Field of the Invention
This invention relates to an optical system adapted for an active type focus detecting device.
2. Description of the Related Art
There have been proposed various infrared automatic focusing devices of the so-called active type having a projection light source and a light receiving element and arranged to scan a photographed object with a light beam coming from the light source, and to continuously or noncontinuously scan the image field with the light receiving element, and to adjust the focus of a photo-taking lens according to an output obtained from a reflected light thus obtained. In the distance detecting device of this kind, the so-called distance measuring visual field is determined by the size of the light source which is projected on the object by means of a light projecting lens. Further, an infrared emitting diode is generally employed as the light source. This element is small in size and is highly efficient. However, since the luminous flux of the emitted light is thin, a spot image resulting from the light projection is small. Therefore, the distance measuring visual field is narrow. This is suitable for accurate focusing on a desired object. However, in cases where there are a plurality of shooting objects spaced at some distance that might allow the spot image of the project light to pass between them, distance measurement cannot be correctly accomplished. Such a distance measuring action results in a so-called middle passing picture having the focus on the background. A method for solving this problem has been proposed, for example, by Japanese Laid-Open Patent Application No. SHO 56-57012 (U.S. Pat. No. 4,371,261). According to this method, an optical element having a plurality of optical axes is employed as the light projecting lens. A plurality of light projection spot images are thus projected on the object from a single light emitting element in such a way as to enlarge the distance measuring visual field without degrading the efficiency of the device.
The optical element which is disclosed by way of example in the above-stated patent application is composed of convex lenses which are cemented together. However, in a case where the optical element is formed by a low-cost injection molding process or the like, the precision of the parts tends to degrade.
Another method for broadening the detection zone has been disclosed in U.S. Pat. No. 4,575,211.
FIG. 7 of the accompanying drawings diagramatically shows the basic arrangement of the active type distance measuring device. Referring to FIG. 7, a light emitting element 1 which is an LED or the like emits either modulated visible rays or infrared rays. A light projecting lens 3 which has a focal length f1 is disposed in front of the light emitting element 1. A light receiving lens 4 which is of a focal length f2 is disposed a base-line length L away from the lens 3. In the rear of the light receiving lens 4 is disposed a light receiving element 2 which is, for example, of a two-area type. A beam of light emitted from the light emitting element 1 passes through the light projecting lens 3 to be projected on an object 5 and thus forms a projected light spot image 6 there. Then, rays of light reflected from the object 5 pass through the light receiving lens 4 to form on the light receiving element 2 an image resulting from the projected light spot image 6. At that moment, the light receiving element 2 is moved in synchronism with adjustment of the focus of a photo-taking lens, which is not shown, in such a way as to make the electrical outputs from two areas of the light receiving element 2 equal to each other. Then, a distance R to the object is detected from a light receiving element moving degree x in accordance with the following formula: EQU R=L.multidot.f.sub.2 /x
In this instance, the distance measuring visual field becomes the size of the projected light spot image 6 obtained by the light projecting lens 3 of the light emitting element 1. Assuming that the size of the light emitting element 1 is 0.5 mm, the focal length f1 of the light projecting lens 3 is 30 mm and the object distance R is 3 m, the size of the projected light spot image 6 is about 50 mm. The light-projecting and light-receiving lenses 3 and 4 are focused either for an object distance selected beforehand or for an infinite distance.